Rendering Enhancements and General Hardening of Embedded Views

Use the learnings from the OutrunXR demo and the QtGraphs porting to ensure we have a good story for embedded and nested application views (2D/3D). Embedding QtQuick/QtQuick3D scene inside the VR should work reliably.

Investigation point from OutrunXR:

• Multiple nested View3Ds in XrItems only shows one of the Views
• There's clear pixelation or specular highlights in the scene. Exact reason needs further investigation.